Ion implanted beams of certain radioactive elements are useful for commercial applications. However, radioactive elements which are not converted to an ion beam may condense inside the vacuum chamber of the ion implanter, particularly in the vicinity of the ion source. Such radioactive deposits may be volatilized as a gas or as a fine airborne dust when the ion source is opened to the air for maintenance, which is a hazardous condition to be avoided.
One useful application of ion beams is to ion implant a radioactive species into surgical components in order to inhibit the regrowth of tissue in a local area. One such component is an intra-arterial stent used subsequent to balloon angioplasty to maintain arterial patency.
Sources of beta radiation ion implanted into the stent wires have been found to inhibit intimal hyperplasia that causes decreased arterial patency. Use of such beta radiation ion implanted stent wires thus reduces the chance of restenosis or reclosure of the artery subsequent to balloon angioplasty or atherectomy The ion implanted atoms cannot be removed accidentally from the component, as might be the case for a coating that peels off, and yet the atoms are positioned near enough to a surface to allow easy exit of the radiation from the component.
An article entitled "Low-Dose, .beta.-Particle Emission from `Stent` Wire Results in Complete, Localized Inhibition of Smooth Muscle Cell Proliferation," by Fischell et al. (the Fischell article) reports the benefits that can be achieved by using phosphorous impregnated stent wire. The Fischell article confirms that the ideal radioisotope to use for coating an arterial stent would be a beta emitter with a half-life of between 10 hours and 100 days. A beta emitter is ideal because the radiation does not penetrate very far through human tissue, thereby reducing the risk of damage to nonproliferating tissue.
Several techniques are available for ion implantation to firmly embed nonradioactive atoms into workpieces. For example, U.S. Pat. No. 4,465,524 by Dearnaley et al. (the '524 patent) describes a method of coating a workpiece made of titanium or an alloy of titanium with a layer of metal, such as tin or aluminum. The metal layer is then bombarded with ions of a species such as nitrogen, carbon, boron or neon, thereby causing the metal to migrate into the titanium. The '524 patent discloses that the modified surface has an improved wear resistance and a reduced coefficient of friction.
Similarly, U.S. Pat. No. 4,831,270 to Weisenberger (the '270 patent) describes an ion implantation system for manufacturing semiconductors. In the ion implantation system of the '270 patent, semiconductor wafers are presented horizontally to an ion beam that impacts the wafers from above. The semiconductor wafers are loaded and unloaded using a wafer transfer station that includes a robot, an evacuatable transporter, and a transfer port that can be sealed with a port on the ion implantation enclosure. The '270 patent indicates that the semiconductor wafers can be doped with boron, phosphorus or arsenic dopant ions. The apparent purpose of the wafer transfer station is to reduce the opportunity for surface particle contamination of the wafers.
U.S. Pat. No. 4,872,922 by Bunker et al. (the '922 patent) describes a method and an apparatus for ion implantation of spherical surfaces using a fixture mounted for motion about two axes normal to each other and a specially adapted work station of an ion beam implanter. The '922 patent discloses using a broad electrostatically scanned ion beam of Ti and C or Ta, which are nonradioactive. The '922 patent indicates that it is important to clean the fixture as well as the spherical workpieces of all surface contamination before operating the ion implanter. The '922 patent suggests that the fixture be cleaned using mechanical abrasion with swabs.
Conventional ion implantation equipment is not designed for use with radioactive materials. The equipment is not adequately radiation shielded, and the need for frequent maintenance of the ion source would be a major safety problem. The ion source of a conventional ion implantation system contains the radioactive feedstock as well as most of the radioactive waste that is not converted to ions.
Useful radioactive species such as phosphorous-32 (.sup.32 P) are particularly hazardous because the ion source only ionizes and accelerates about 1% of the phosphorus vapor produced. The remaining phosphorus vapor is generally condensed on cold surfaces near the ion source as white phosphorus, a solid form of elemental phosphorus which has a significant vapor pressure at room temperature and is well-known to be strongly reactive with oxygen. White phosphorus is a known airborne health hazard even when it is not radioactive. The maximum allowable concentration in air is 0.1 mg/m.sup.3. The oxides produced by the spontaneous burning of white phosphorus are also highly toxic chemically.
An ion source intended to produce a radioactive .sup.32 P ion beam will concurrently discharge into the vacuum system significant quantities of unused white phosphorus in a mixture of non-radioactive .sup.31 P and radioactive .sup.32 P. When the ion source requires regular servicing, for example to add fresh radioactive feedstock or to replace a consumable component such as a tungsten cathode, the phosphorus coated walls can release radioactive gas into the environment, creating a major safety hazard.
An ion source that has been creating phosphorus vapor also emits a pungent odor when opened to air. In addition, if a vacuum system is raised to atmospheric pressure or pumped to vacuum from atmosphere, even if an inert gas is employed, a significant wind exists until the vacuum system reaches pressure equilibrium. Such a wind disperses radioactive particles throughout the system and coats radioactive dust on surfaces that would not normally be contaminated. Therefore, use of radioactive species such as .sup.32 P in an ion implanter is a difficult safety problem due to airborne radioactive vapors and particles.
There are fewer conventional techniques for ion implantation using radioactive ions than there are for nonradioactive ions. One example of a technique for ion implantation of a radioactive ion beam is U.S. Pat. No. 4,124,802 by Terasawa et al. (the '802 patent), which is directed to a method and device for ionizing radioactive gas such as Kr-85, accelerating the ionized radioactive gas into a high energy form, then implanting the high energy radioactive gas in a base material such as a band-shaped stainless steel foil. The radioactive ion implantation technique described in the '802 patent is designed for use in waste disposal of radioactive gases from nuclear reactors.
The ion implantation system of the '802 patent to Terasawa et al. for Krypton-85 is simple in its maintenance requirements for the ion source because unused radioactive feedstock gas can be almost completely removed from the ion implanter by the pumping/recovery system since it is an inert, non-condensible gas. Thus, the ion source retains essentially no radioactivity and any radioactivity that is present is not likely to be airborne during maintenance. However, the ion implantation system of the '802 patent is not optimal for a source that is not a gas at room temperature, such as phosphorous. Waste feedstock materials of phosphorus, for example, could condense as a solid phosphorus coating inside the ion implanter, making it more difficult to repair and maintain the ion source. Such radioactive phosphorus residue is an unsealed radioactive source, and phosphorus can readily become airborne as a gas or dust.
U.S. Pat. No. 5,059,166 by Fischell et al. (the '166 patent) describes a technique for causing a helical spring stent to be radioactive prior to insertion into the artery. The '166 patent discusses various techniques for causing the stent to be radioactive, including using radioisotopes in the manufacture of the stent and/or plating the stent with a radioisotope coating. Although the '166 patent generally describes a process for making the stent radioactive, it may be assumed that using radioisotopes and/or applying a radioactive coating would require handling of radioactive material in a manner which may be unacceptable for mass production purposes.
An alternative approach for creating a radioactive stent involves using a cyclotron to bombard stainless steel stents with a proton beam to produce radioactive isotopes within the stent. However, these isotopes have high-energy gamma emissions and long half-lives that make this technique impractical for humans. Gamma emissions increase the whole-body dose of radiation while having relatively little therapeutic effect on local tissue compared to the effect of beta emissions. In addition, it has been found that long half-life materials are less appropriate since optimum radioactivity-mediated inhibition is more likely to be achieved by a continuous exposure for the first few weeks following the insertion of the stent. Accordingly, a half-life of a few weeks (i.e., one to seven weeks) is deemed ideal for this purpose.
Another technique for manufacturing radioactive stents involves first implanting massive doses of .sup.31 P in titanium stents. The .sup.31 p is subsequently activated by a nuclear reactor to produce .sup.32 P. However, this technique requires up to thirty atomic percent .sup.31 P under the surface of the titanium stent, which alters the chemical composition of the alloy with unpredictable effects on the mechanical and biocompatibility properties of the stent. In addition, the titanium metal stent may have impurities which, when bombarded in a nuclear reactor, create isotopes that emit substantial gamma rays and have long half-lives similar to the isotopes in the stainless-steel stents that are bombarded in a nuclear reactor.
An article entitled "Production and Quality Assessment of Beta Emitting P-32 Stents for Applications in Coronary Angioplasty", by Janicki et al. (the Janicki article) discusses using ion beam implantation to implant radioactive .sup.32 P isotopes into a titanium stent. Although the ion beam implantation technique itself appears to result in a radioactive stent having acceptable characteristics, the Janicki article discloses performing the ion beam implantation using radioactive cathodes that are prepared using radioactive salts that are first dissolved into liquids and then dried onto the cathode. Handling the thus-formed radioactive liquids may be unacceptable for mass production. In addition, the resulting radioactive salts that are dried onto the cathode may fall off and contaminate workers in a mass production setting.